UnintentionalChaos
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Phosphate/Arsenate testing solution
In my research position, I'm doing work that involves arsenates and phosphates and today, we prepared a reagent that converts phosphate and arsenate
solutions to phosphomolybdic acid and phosphoarsenic acid solutions. (I believe that these complexes are not the phosphomolybdic or arsenomolybdic
acid species per-se, but ones with lower oxides of molybdenum substituted in for some of the original molybdates).
The preparation and testing is as follows:
Prepare a solution containing approximately 1mg/mL of antimony. We used potassium antimony tartrate.
Prepare a 4% ammonium molybdate solution (4g ammonium molybdate hydrate into a 100mL volumetric flask). This should be stored in the freezer when not
being used.
Prepare a crudely 20% solution of sulfuric acid (we just used 20mL of concentrated H2SO4 in a 100mL volumetric, which was close enough).
Combine 50mL of the H2SO4 with 5mL of the antimony solution and 15mL of the molybdate solution. Dilute with 30mL of deionized water and keep this
solution in the fridge. I don't think this is shelf stable and should be made as needed.
Prepare a 5% ascorbic acid solution (5g into a 100mL volumetric flask)
Take your sample of phosphate or arsenate solution (our samples to be tested were roughly 40uM).
To 4mL of sample, add about 0.25mL of the ascorbic acid solution. Shake, then add 0.75mL of acetone and shake again. Add 1mL of the mixed reagent
prepared above and shake. Blue color develops rapidly.
I'm thinking this has woelen written all over it 
I suspect that the antimony is nothing but a catalyst. Hell, the 100mL of mixed reagent contains about 5mg of antimony ions. I feel like preparing the
standardized solution is probably an unnecessary step, and only required for standardization of the procedure.
I did a bit of googling and it appears that the blue product is a reduced form (with some of the molybdate reduced, I believe) of the typical yellow
pnictogenomolybdic acids.
A comparison with and without the ascorbic acid is probably in order, but I don't have any molybdates around at home.
[Edited on 6-22-09 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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DJF90
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Thats a nice contribution UC, thanks alot. I dont intend on working with anything arsenic but the phosphate test is definately nice.
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JohnWW
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That is the basis of the standard quantitative colorimetric or spectrophotometric method for the determination of phosphate in aqueous solution, via
the formation of the intensely yellow ammonium phosphomolydate charge-transfer complex, which has its peak absorption in the blue-violet part of the
spectrum. However, any soluble salts of arsenate and antimonate would interfere if present, as would probably also the likes of stannate, plumbate,
bismuthate, etc.. See "Standard Methods For The Examination Of Water & Wastewater", and "Vogel's Textbook Of Quantitative Inorganic Analysis",
links for which are in the references section.
[Edited on 22-6-09 by JohnWW]
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bfesser
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Is there a way to prepare a similar indicator reagent using ammonium molybdate, but without antimony? What is the function of the antimony?
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pantone159
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Quote: Originally posted by UnintentionalChaos  |
Prepare a 4% ammonium molybdate solution (4g ammonium molybdate hydrate into a 100mL volumetric flask). This should be stored in the freezer when not
being used. |
You keep this 4% solution as a frozen solid??
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woelen
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This indeed is a very interesting reaction. I also have found that the formation of the deep blue lower oxidation state compound of molybdenum is
catalyzed by the presence of phosphate ion. Where molybdate alone may be difficult to reduce with mild reductors, in the presence of phosphate this is
much easier. Probably the mixed molybdophosphate complex has such a structure that the molybdenum is more easily reduced.
A similar thing I have seen with the presence of hydrochloric acid. In the presence of HCl there is more easy reduction of molybdate. Just try it with
mild reductors like sulfite, thiosulfate and ascorbic acid. Strong reductors like powdered zinc or dithionite reduce the molybdate anyway.
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bfesser
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Ascorbic works nicely to reduce both molybdate and tungstate to colored species. I'd assume that it works on chromate, as well. Kinda makes one
wonder about 'seaborgate'... if it exists.  (Please don't respond to that and
drag the thread off topic.) Still, it would be nice to avoid the antimony--time to start scanning the literature.
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